The present invention relates to a semiconductor memory device, and more particularly to techniques which are effective when applied to a semiconductor memory device wherein memory cells are arranged at the intersectional parts between data lines and word lines.
In a semiconductor memory device, for example, a dynamic random access memory (hereinbelow, abbreviated to "DRAM"), the signal waveforms of data lines, word lines, a reference clock signal line for RAS-group circuits, etc. are observed (measured). The observation is made in order to judge whether or not the DRAM is functioning as designed.
The observation is usually made with a probe tester by interposing an observational MOSFET between an observational pad and the signal line to-be-observed. In particular, the data line presents the signal waveform whose information is a minute amount of charges, so that the observational MOSFET is interposed lest the signal waveform should change due to the stray capacitance and resistance of a probe. The observational MOSFET has the signal line to-be-observed connected to its gate electrode, has a power source voltage (reference voltage) connected to its drain region, and has the observational terminal (pad) connected to its source region. The probe of the probe tester is brought into touch with the measuring terminal. That is, the observation is made in such a way that the operation of the observational MOSFET is controlled by the change of the signal waveform of the signal line, whereupon a voltage developed at the measuring terminal by the operation is detected by the probe. The observational MOSFETs and measuring terminals as stated above are arranged by utilizing so-called dead spaces such as the ends of the data lines, the ends of the word lines and the interspaces between circuits.
It is recently practiced to observe the signal waveforms of the data lines, the word lines etc. by the use of an electron beam (hereinbelow, abbreviated to "EB") tester. The observation with the EB tester features that, since the projection diameter of an electron beam is as small as about 0.1 [.mu.m] by way of example, the signal waveform can be observed by directly projecting the electron beam on the signal line.
Incidentally, the EB tester is stated in, for example, NIKKEI MICRODEVICE, January 1987, pp. 34-37.
Besides, regarding a logic LSI which has multilayer wiring, means for effectively measuring the potential of a lower wiring layer with a low-acceleration SEM has been applied for a Japanese patent by the same assignee as in the present application (Japanese Patent Application No. 56-96639).
The specification of the above Japanese patent application states to the effect that the lower wiring layer is led out via the through hole of an inter-layer insulator film to an overlying observational pad, at which the signal waveform of the interior can be observed.